1. Technical Field
This invention relates to flow heaters for heating liquids, e.g. water.
2. Background Information
One form of conventional flow heater comprises a channel having a heating element on its outside, which allows overheat protection for the heating element to be provided in close thermal contact with the element, so that the element can be switched off quickly and effectively in the event of it being energized without water in the channel to prevent it from overheating.
However, such high power flow heaters are prone to limescale build up in the relatively narrow channel, which can both restrict the flow of water and insulate the heating element. As will be appreciated this insulation of the heating element and restriction of the water flow causes the heating element to operate at an increasingly higher temperature during normal use which will eventually lead to the overheat protection operating to disconnect the heater even when water is flowing through the channel. This is known in the art as “Dry Boil Interference” (DBI). When such a situation is reached it is not possible to operate the flow heater as the overheat protection will switch off the element before the required volume of water has been heated. Furthermore, it is not possible to design around this situation, e.g. by increasing the temperature at which the overheat protection operates, as this would raise the threshold temperature to a dangerously high value, i.e. one at which genuine overheat (e.g. with little or no water in the flow heater) is not detected quickly enough resulting in permanent damage to the appliance and/or a risk of fire. Such heaters may therefore require regular de-scaling, especially in hard water areas.
It is an object of the invention to provide a flow heater with an improved overheat protection arrangement.